The present invention relates to a gun configured to generate charged particles.
Nano-size electron emitters (hereinafter nano-emitters) are typically used in some electron gun designs. A type of electron gun design uses a conventional single-tip cathode emitter 100 (i.e. see FIG. 1a), which is commonly adopted for electron microscopy. This type of electron gun design is characterised with smaller effective source sizes, and brightness of one to two orders of magnitude higher than electron guns using larger single-tip emitters, but is however disadvantaged by proportionally lower total beam currents (which are typically in the pico-ampere to tens of nano-ampere range, and thus not useful for Electron Beam Lithography and analytical techniques such as Auger Electron Spectroscopy (AES)), large beam current fluctuations, and more stringent vacuum-operating requirements (i.e. less than 10-10 Torr). In addition, there are associated alignment problems relating to cathode-tip to rotational axis of subsequent lenses.
In another type of electron gun design, the nano-emitters are arranged as an array covering a large area (i.e. thus also known as a large area field emitter array), and the nano-emitters are usually made from CNTs. Large area field emitter arrays have several advantages over the single-tip cathode emitter 100. Specifically, large area field emitter arrays can be manufactured using standard Chemical Vapour Deposition (CVD)/lithography techniques, wherein tens of thousands of nano-emitters can routinely be formed over an area that cover up to several centimeter squared (i.e. cm2), and also large area field emitter arrays are able to output a maximum electrical current density of about 1 A/cm2. Take for instance vertically aligned carbon nano-tube field emission arrays, which typically include sub-micron diameter CNT blocks spaced apart by a few microns, and each CNT block is formed from tens of thousands of nanometer-size CNTs. In this respect, previous studies report obtaining a total probe current of between 0.3 μA to 300 μA from a large area field emitter array configured with hundred by hundred dots, where each dot is a 5 μm high CNT fibre with a diameter of 100 nm at the base of, and 50 nm at the tip of the CNT fibre. It is to be appreciated that the above said level of the probe current reported is typically orders of magnitude higher than a probe current obtainable from a single-tip cathode emitter 100. Moreover, the problem of ion back-bombardment is expected to be much smaller with large area field emitter arrays, due to ions being distributed over a much bigger cathode surface, which means that a large area field emitter array only requires an operating vacuum level of about 10−6 Torr, being considerably several orders of magnitude lower than that required by conventional cold field emitters.
However, large area field emitter arrays are not suitable for focused beam applications, due to them having a relatively large source. Conventionally, large area field emitter arrays are adopted for applications only where a target area is at least greater than several millimeter squared (i.e. mm2) (e.g. in flat panel displays or X-ray tubes).
One object of the present invention is therefore to address at least one of the problems of the prior art and/or to provide a choice that is useful in the art.